A matched filter is a filter for judging the identification between two signals. In spread spectrum communication, each user who receives a signal processes the received signal by a matched filter using spreading code allocated for the user to find a correlation peak for acquisition and holding.
Here, assuming that the spreading code is d(i), sampling interval is .DELTA.t, a length of diffusion code is N, a received signal before a time t is x(t-i.DELTA.t), and a correlation output y(t) of matched filter is as in formula (1). In formula (1), d(i) is a data string of 1 bit data. ##EQU1##
A conventional matched filter circuit is described herein. In a digital circuit in FIG. 14, digitized input signal X is held in a shift register SFT-REG and shifted, then, a multiplier registered in a register REG is multiplied to a input signal on the predetermined sample timing by a plurality of digital multiplying portions DM. The outputs of multiplying portions are added by a digital adder DAD. These operations correspond to the formula (1). For the acquisition, double or higher order of sampling is necessary. In such a case, a structure with many circuits of FIG. 14 are required. Consequently, the size of the whole circuit to large and has high electric power consumption. It is a serious defect. Though a circuit of SAW device was used, the total circuits cannot be incorporated within one LSI and make S/N ratio low.
The applicant of the present invention proposes a matched filter circuit by an analog circuit in FIG. 15 in Japanese publication patent numbers 06-2839070, 06-296120, 06-314952 and 07-6190. The Electric power consumption was reduced by a circuit with a multiplier MUL and an adder ADDER of voltage driven type using a capacitive coupling. However recently, portable communication terminal is expected to reduce further electric power consumption.